222 Contributions to Electricity and Magnetism. 
53. I may also mention that the energetic action of the spiral 
conductors enables us to imitate, in a very striking manner, the 
inductive operation of the magneto-electrical machine, by means 
of an uninterrupted galvanic current. For this purpose, it 1s only 
necessary to arrange two coils to represent the two poles of a 
horse-shoe magnet, and to cause two helices to revolve past them 
in a parallel plane. While a constant current is passing through 
each coil, in opposite directions, the effect of the rotation of the 
helices is precisely the same as that of the revolving armature In 
the machine. 
54. A remarkable fact should here be noted in reference to he- 
lix No. 4, which is connected with a subsequent part of the in- 
vestigation. This helix is formed of copper wire, the spires of 
which are insulated by a coating of cement instead of thread, as 
in the case of the others. After being used in the above expel 
ments, a small discharge from a Leyden jar was passed through 
it, and on applying it again to the coil, I was much surprised to 
find that scarcely any signs of a secondary current could be ob- 
taine _ : 
55: The discharge had destroyed the insulation in some part, 
but this was not sufficient to prevent the magnetizing of a bar of 
iron introduced into the opening at the centre. ‘The effect ap 
peared to be confined to the inductive action. The same accl 
dent had before happened to another coil of nearly the same kind. 
It was therefore noted as one of some importance. An explana 
tion was afterwards found in a peculiar accident of the secondary 
current.* 
SECTION IV. 
On the Effects produced by interposing different Substances Oe 
= tween the Conductors. 
56. Sir H. Davy found, in magnetizing needles by an electric 
discharge, that the effect took place through interposed plates © 
all substances, conductors and non-conductors.t The expetir 
ment which I have given in paragraph 51 would appear ine 
cate that the inductive action which produces the secondary cur: 
rent might also follow the same law. 
st te A ce a 
* See paragraph 75. + Philosophical Transactions, 1821. 
